What's the purpose of foam in a sealed speaker?

Filius

Filius

Member
My Celestion 100s have sealed cabinets and are stuffed with foam. I mean not just lining the walls of the inside of the cabinet, but pretty much totally packed with it.

I have heard of using polyfill or fiberglass inside of sealed cabinets in order to "fool" the woofer into thinking it's in a bigger space, something to do with the fibers rubbing together and generating heat. But what does stuffing them with foam accomplish?
 
Foberous materials all work in the same way though there is a large in how well various materials work. The fibers rub together when the speaker move the air in the cabinet so mechanical acoustical energy is turned into and dissipated as heat. Open cell foam works by friction also but it is the friction of the air in all the small cell openings. My klipsch Heresy 3 are heavily braced and are about 95 % totally stuffed with high density fiberglass. Good old fiberglass especially high density is about as goo a damper as there is and is more effective over a much wider bandwidth than poly fill or foam. Best regards Moray James
 
moray james said:
Foberous materials all work in the same way though there is a large in how well various materials work. The fibers rub together when the speaker move the air in the cabinet so mechanical acoustical energy is turned into and dissipated as heat. Open cell foam works by friction also but it is the friction of the air in all the small cell openings. My klipsch Heresy 3 are heavily braced and are about 95 % totally stuffed with high density fiberglass. Good old fiberglass especially high density is about as goo a damper as there is and is more effective over a much wider bandwidth than poly fill or foam. Best regards Moray James
Click to expand...
Is there anymore on this topic, about heat and why this works this way?
 
So foam or fiberglass in a sealed speaker dampens the frequencies resulting in less cabinet material vibrating (creating a decrease in unwanted freqs) and a better listening experience because we will hear an increase in actual sound coming from the speaker itself, less from the enclosure…

Is my understanding correct? :scratch2:
 
There are two independent things that fiberous and foam fillings do in an enclosure. One is that they are an acoustical resistance and therefore absorb acoustic energy significantly reducing standing wave mode resonances in the box - usually at midbass frequencies for large bookshelf speakers. They also partially, and sometimes poorly, absorb all frequencies and perhaps reduce re-radiation due to reflections in the box and back out the cone, but this is not often a primary issue. Standing wave modes are powerful and cause the walls to flex more than anything else and therefore reducing standing wave energy will indirectly reduce wall flex.

These materials also act as a heat sink for the air taking it from an adiabatic system to iso-thermal. This has been extensively studied in thermodynamics simply to understand the nature of gases. The compliance of air goes up, ideally, by 1.4 when the system moves from adiabatic to iso-thermal. This is not disputed by the majority of audio professionals or thermodynamics experts but there are a few that have made absurd claims to the contrary. Here is the basic gas law theory:
http://farside.ph.utexas.edu/teaching/sm1/lectures/node53.html

Note the mention of a "heat reservoir" this is essentially a heat sink that keeps the gas at a constant temperature. The fiberous, or porous, materials have a very large surface area and they act as a thermal "mass", "heat reservoir" or heat sink keeping the gas at a constant temperature.

The compliance or effective box size ideally goes up by a factor of 1.4 with complete filling. The fundamental resonance is not linear with compliance so one should only expect resonance to go down by 1/sqrt(1.4).

Years ago I did not think that foam had enough surface area to be as effective as fiberous materials but I was wrong, it is actually quite good. All of these materials have different frequency dependent acoustic impedances making some better than others in particular applications.

Fibers moving is a second order effect and has little to do with the above concepts as I understand it.

Tom Nousaine's "Filler Up" Article in .pdf: http://www.nousaine.com/pdfs/Box Stuffing.pdf
 
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In a sealed speaker, closed cell foam will make the box act smaller then it is, the foam takes away volume.
This is good if the box is just about the right size for the woofer or too big.

If the box is too small, it can be made to act bigger, by stuffing it with rockwool or fiberglass.
As far as I remember it is possible to increase effective volume by up to 40% by stuffing the box good, but it's probably better to expect less then that.
 
Pete B said:
There are two independent things that fiberous and foam fillings do in an enclosure. One is that they are an acoustical resistance and therefore absorb acoustic energy significantly reducing standing wave mode resonances in the box - usually at midbass frequencies for large bookshelf speakers. They also partially, and sometimes poorly, absorb all frequencies and perhaps reduce re-radiation due to reflections in the box and back out the cone, but this is not often a primary issue. Standing wave modes are powerful and cause the walls to flex more than anything else and therefore reducing standing wave energy will indirectly reduce wall flex.

These materials also act as a heat sink for the air taking it from an adiabatic system to iso-thermal. This has been extensively studied in thermodynamics simply to understand the nature of gases. The compliance of air goes up, ideally, by 1.4 when the system moves from adiabatic to iso-thermal. This is not disputed by the majority of audio professionals or thermodynamics experts but there are a few that have made absurd claims to the contrary. Here is the basic gas law theory:
http://farside.ph.utexas.edu/teaching/sm1/lectures/node53.html

Note the mention of a "heat reservoir" this is essentially a heat sink that keeps the gas at a constant temperature. The fiberous, or porous, materials have a very large surface area and they act as a thermal "mass" or heat sink keeping the gas at a constant temperature.

The compliance or effective box size ideally goes up by a factor of 1.4 with complete filling. The fundamental resonance is not linear with compliance so one should only expect resonance to go down by 1/sqrt(1.4).

Years ago I did not think that foam had enough surface area to be as effective as fiberous materials but I was wrong, it is actually quite good. All of these materials have different frequency dependent acoustic impedances making some better than others in particular applications.

Fibers moving is a second order effect and has little to do with the above concepts as I understand it.

Tom Nousaine's "Filler Up" Article in .pdf: http://www.nousaine.com/pdfs/Box Stuffing.pdf
Click to expand...
Awesome info. So, by keeping temp at a constant, that keeps internal "pressure" (effectively) at a constant as well? I'm assuming that as temperature rises so does effective pressure inside the enclosure. But what about thermal heat generated by the woofer VC? All these things have an impact on efficiency, correct?
 
Copa1934 said:
Awesome info. So, by keeping temp at a constant, that keeps internal "pressure" (effectively) at a constant as well? I'm assuming that as temperature rises so does effective pressure inside the enclosure. But what about thermal heat generated by the woofer VC? All these things have an impact on efficiency, correct?
Click to expand...

A sealed box should be slightly leaky, so that weather and temperature variations doesn't cause high or low pressure in the box compared to outside of it.
 
The cone moves in, pressure goes up, then temp goes up causing pressure to go up even more, add filling and it holds the temp closer to constant so the second increase is reduced and the box acts larger. Cone moves out pressure drops, temps drops ... same idea.

VC heating causes a slight rise in air temp only, it is like a DC term but most do not take it into account. Since it is a very slow rise, you could do one simulation of the enclosure at the expected room temp to show low level listening, then compute or assume some rise due to high level listening and adjust the air temp in the simulation. I've never heard of anyone doing this.

Acoustical losses due to the stuffing will lower the box Q but it is dependent on the type of material used and how close it is placed to the woofer cone.
Material very close to the cone will lower the effective Qms of the driver.
Some believe that fiberglass more than others adds some acoustical mass when placed very close to the woofer cone and this will also lower Fc and Qtc for closed box systems.

Vented systems are very sensitive to stuffing and the old rule was to line the walls with .5 to 1", many are using much more but port output usually suffers often by as much as 3 to 6 dB or more. The smarter thing to do is to add more at points away from the port and driver, and also to use a material that is less effective as an acoustical resistance at box resonance. The old Gensis II for example is passive radiator based and completely stuffed with fiberglass - it is surprising that the bass is so good. Also, standing waves in the box have velocity minima at the boundaries, obviously no air can move at the wall of the enclosure and maxima toward the center of the box. Damping materials are most effective at velocity maxima so lining the walls is far from the best solution but it is better than nothing.
 
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Adda said:
In a sealed speaker, closed cell foam will make the box act smaller then it is, the foam takes away volume.
This is good if the box is just about the right size for the woofer or too big.

If the box is too small, it can be made to act bigger, by stuffing it with rockwool or fiberglass.
As far as I remember it is possible to increase effective volume by up to 40% by stuffing the box good, but it's probably better to expect less then that.
Click to expand...

Open cell acoustical foam does not take away effective volume rather it increases it. See testing in the LDC for proof.
 
Adda said:
If the box is too small, it can be made to act bigger, by stuffing it with rockwool or fiberglass.
As far as I remember it is possible to increase effective volume by up to 40% by stuffing the box good, but it's probably better to expect less then that.
Click to expand...

In his "Loudspeaker Handbook" the late JBL engineer John Eargle said that in practice stuffing made sealed boxes act only slightly larger, less than 20% as I recall. I wish I still had the book to be sure. In any event I was surprised at how small an effect it had on apparent volume.
 
Tom Brennan said:
In his "Loudspeaker Handbook" the late JBL engineer John Eargle said that in practice stuffing made sealed boxes act only slightly larger, less than 20% as I recall. I wish I still had the book to be sure. In any event I was surprised at how small an effect it had on apparent volume.
Click to expand...

It probably depends on what you stuff with, I too think that 40% seems high.
 
They are wrong, look at Nousaine's info and the LDC info.
Nousaine says "enjoyed a 25 to 35% increase" an average of 30 is certainly better than 20%
and no one should expect the ideal 40%.
They probably miscomputed expecting Fc to be linear with compliance - which it is not.
 
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